A known example of a typical oil seal is a two-fluid oil seal provided between an outer circumference of an engine crankshaft and an inner circumference of a flywheel housing. The two-fluid oil seal includes a pair of lips that seal an engine side and a flywheel side to prevent leakage of two oils, such as an engine oil supplied to the crankshaft and a lubricating oil supplied to components of the flywheel (e.g., a clutch disc and a transmission), from one side to the other side.
The lips of the two-fluid oil seal are each provided with a helical groove on a surface in contact with the crankshaft so that an oil that is leaking into a space between the lips is discharged out of the space. When the oil is leaking into the space between the lips, the rotation of the crankshaft makes the oil flow into the groove, and then the oil is helically discharged outside as the crankshaft rotates.
However, since air in the space is also discharged along with the oil discharged out of the space, the pressure in the space becomes negative. The lips are thus forced against the crankshaft, thereby accelerating wear of the oil seal. As a result, the sealing performance and durability of the oil seal may be deteriorated. Accordingly, the two-fluid oil seal employs an arrangement where one of the lips (e.g., a flywheel-side lip) is provided with a pressure release hole (see, for instance, Patent Literatures 1 to 3).
An oil seal disclosed in Patent Literature 1 includes a first seal that seals a flywheel side and a second seal that seals a crankshaft side. The first seal is provided with a communication hole through which a space surrounded by the first seal, the second seal and the crankshaft is in communication with a space inside a flywheel housing.
An oil seal disclosed in Patent Literature 2 similarly includes a pair of seal members that are brought into contact with a crankshaft to isolate first and second lubricating oil chambers from each other. One of the seal members of this oil seal is provided with a communication passage through which a space of one of the isolated spaces and an annular gap between the seal members are in communication with each other.
An oil seal disclosed in Patent Literature 3 includes a pipe through which a space between a pair of lips and a flywheel-side space (a mission room) are in communication with each other.